1. Field of the Invention
The present invention relates generally to laser printers, copiers and all-in-one devices, and more particularly to toner fusers for use with such printers.
2. Description of the Related Art
In laser type printers, toner particles are electrostatically attracted to a print media, such as paper, to produce text, symbols or other images. The toner particles must be fused to the paper in order to make the text or image permanent and resistant to smudging or smearing. Once the toner particles are electrostatically attracted to the print media in the pattern of the text or image, the toner is fused to the print media through the use of high temperatures and pressure applied to the toner in order to permanently imbed the toner into the print media, or on the surface thereof. As can be appreciated, the temperature to which the toner is subjected must be controlled in order to assure a consistent, quality print job. The fusing temperature can change as a function of changes in parameters such as paper weight, duration of printing, etc. Fusing temperatures higher than necessary can cause the toner to fuse to some of the printer apparatus, such as the fuser belt. A fusing temperature that is too low will result in inadequate fixing of the toner to the print media, thus allowing smearing of the text or image. The fusing temperature of the toner must thus be automatically adjusted in order to maintain an optimum print quality.
Several types of fusers are known in the art for fixing the toner particles to the print media. One type of fuser employs an axial lamp to generate the power necessary to fuse the toner to the print media. Another type of fuser includes a flat ceramic slab heater with a heating resistor on one side of the ceramic member and a thermistor on the back side of the heater member. The thermistor functions to sense the temperature of the ceramic member and through feedback control circuits, control the electrical energy applied to the heating resistor and thereby control the temperature at which toner fusing occurs. Typically, the heater comprises one or more strips of a metallic material that becomes heated when an electrical current passes therethrough. Generally, the heater strip is driven by a source of AC electricity that is controlled in some manner in order to control the amount of thermal energy generated by the heater strip.
The use of the ceramic slab type of fuser heater is advantageous as there is a high speed transfer of heat from the heater, through the ceramic slab, to the print media. The faster the heat can be transferred to the print media, the more quickly the printer can commence printing. The time many printers must wait until the fuser is sufficiently hot to commence printing the first sheet of print media can be 10-20 seconds. The faster the printer can start printing, the more efficient the printer becomes. The ceramic slab heater technology is often referred to as “instant on” fusing technology. This is desirable because the time to first print is on the order of 10 seconds, or less.
In order to efficiently transfer thermal energy, the ceramic slab of the heater is constructed as a thin member, on the order of about 0.5 mm to about 2.0 mm. This construction facilitates a low thermal mass and thus can be quickly heated to the desired operating temperature. The thin construction of the ceramic slab permits the temperature sensor to be located in close proximity to the heated side of the slab heater, thereby permitting more accurate control of the temperature, and shorter response times to reach desired fuser temperature.
As noted above, the heating element located on one side of the ceramic slab heater is normally driven by the AC line voltage having a magnitude of either 120 Vrms or 240 Vrms. The thermistor is located on the opposite side of the ceramic slab heater, and is connected to low voltage circuits (5VDC) of the printer fuser. Accordingly, both the AC power line voltage circuits and the low DC voltage circuits exist in close proximity to each other in the fuser assembly. The thin ceramic slab member functions as an electrical insulator between the AC and DC circuits. As can be appreciated, it is highly desirable to maintain adequate electrical isolation between the AC power line circuits and the DC circuit of the printer, otherwise substantial damage can occur if the AC line current is allowed to be imposed on the DC circuits of the printer. A failure in the ceramic slab, such as cracking thereof, can allow the AC energy of the heater apparatus to be effectively connected to the low voltage DC circuits of the fuser. Unless isolated, the AC energy can propogate from the DC circuits of the fuser to the other down line circuits of the printer. As noted above, the AC and DC circuits of the fuser are separated from each other by only 1 mm, or so, i.e., the thickness of the ceramic slab.
Various AC/DC isolation schemes for fusers have been proposed in the laser printer field. One electrical isolation technique involves the use of a 1:1 transformer to isolate the thermistor from the AC line voltage. See Electronic Design magazine, Apr. 22, 2008, article entitled “Circuit Transfer Resistance Value through Isolation Barrier,” by Leo Sahlsten. With this technique, the magnetic coupling between the primary and secondary of the transformer provides the electrical isolation. The disadvantage with this method is that the scope of the resistance change in the thermistor is small, namely about 1.5 decades of resistance change. This obviously limits the accuracy and/or overall range by which temperature changes can be sensed. A much better AC/DC isolation technique would allow detection of 4+ decades of thermistor resistance change, namely from about 2.4E6 Ohms (cold or nearly open) to about 2.1E3 Ohms (over temperature).
Another technique for providing electrical isolation between the AC and DC circuits of the fuser of a laser printer involves the use of capacitive coupling therebetween. With this technique, a capacitor, such as a Y-cap, is allowed to be connected between the AC line voltage and the low voltage DC circuits. Again, this method fails to allow 4+ decades of resistance change in the thermistor to be accurately and efficiently detected.
Yet another method of providing AC/DC isolation in printer fuser apparatus involves the use of optical techniques to couple light energy to the thermistor. While this provides ideal electrical isolation between the AC circuits and the DC circuits, the problem is the inability to effectively provide sufficient electrical energy to power the thermistor in order to obtain a voltage therefrom representative of the temperature. Stated another way, there is insufficient power available in the optical signals so that when transferred through an opto-isolator, the resulting electrical energy will not adequately energize the thermistor.
From the foregoing, it can be seen that a need exists for a printer fuser that can provide sufficient DC power to power the circuits thereof, and provide output electrical signals representative of the fuser temperature, all electrically isolated from other DC circuits of the printer. Another need exists for a printer fuser that is isolated from the other circuits of the printer by a separate ground system. Yet another need exists for an improved printer fuser employing temperature conversion circuits where multiple decades of temperature changes can be converted into multiple decades of resistance values to thereby provide increased temperature measuring accuracy.